Oobleck

Oobleck is a suspension of cornstarch and water that can behave like a solid or a liquid depending on how much pressure you apply. Try to grab some in your hand, and it will form a solid ball in your palm until you release the pressure. Then, it will flow out between your fingers. Materials that behave this way are classified as non-Newtonian fluid because their flow properties are not described by a constant viscosity. The name Oobleck comes from the 1949 children’s book, Bartholomew and the Oobleck, by Dr. Seuss. In the story, a sticky liquid falls from the sky as a result of the king becoming bored with normal weather.

Here’s what you need

Let’s get to the fun stuff. In order to make a Oobleck you will need the following items:

Cornstarch

Pitcher of Water

Aluminum pie pans

Measuring cups

Mixing spoon

Newspaper for covering tables

Food coloring or tempera paint (for fun)

What to do

Place one and a half cups of cornstarch in a pie pan

Add one cup of water, and stir well.

Add more water or cornstarch until you get a mixture that ‘tears’ when you quickly scrape your finger through it and then ‘melts’ back together again.

Squeeze it…Squish it…Scoop it…Pinch it…What happens?

If you want to color your Oobleck, add some tempera paint. You can use food coloring if that’s all you have on hand. Food coloring tends to stain more than the paint, especially if you have a spill while preparing your Oobleck.

One thing to keep in mind is that Oobleck is a suspension, not a solution. The cornstarch does not dissolve in the water like salt or sugar would. Instead, the tiny starch particles are suspended in the liquid. If you let it sit long enough in a glass, the cornstarch will settle to the bottom leaving a layer of clear water on the top. This is why it is very important not to pour Oobleck down the drain. Should the suspension separate in your drain pipes, you will be left with a hard clump of cornstarch that will block the drain. The best way to get rid of your Oobleck is to simply put it in your trash can.

What does non-Newtonian mean?

All fluids have a property known as viscosity that describes how the fluid flows – commonly thought of as how thick or thin a fluid is. For instance, honey is much more viscous than water. When a fluid’s viscosity is constant, it is referred to as a Newtonian fluid. Oobleck is an example of a fluid whose viscosity is not constant; it’s viscosity changes depending on the stress or forces applied to it. If you poke it with your finger and apply a large force, it becomes very viscous and stays in place. If you gently pour it, applying little force, it will flow like water. This kind of fluid is called a dilatant material or a shear thickening fluid. It becomes more viscous when agitated or compressed.

Another non-Newtonian liquid is ketchup. Ketchup behaves in the opposite way from Oobleck. You could even call it the “anti-Oobleck.” It becomes less viscous when agitated. Liquids like this are called shear thinning liquids. If you leave a bottle of Ketchup on a shelf, it becomes thicker or more viscous. Nearly everyone has experienced this while trying to pour the liquid from a new bottle – it refuses to move. If you shake the bottle or stir it up, it becomes less viscous and pours easily.

Why does Oobleck behave the way it does?

The most generally accepted explanation for the behavior of Oobleck is offered by Cary Sneider in “Oobleck: What do Scientists Say?”. When sitting still, the granules of starch are surrounded by water. The surface tension of the water keeps it from completely flowing out of the spaces between the granules. The cushion of water provides quite a bit of lubrication and allows the granules to move freely. But, if the movement is abrupt, the water is squeezed out from between the granules and the friction between them increases rather dramatically.

Experiments to try

The first thing you have to do is simply place your hands into the Oobleck and start squeezing it. Have some fun! Try to make a ball by moving it around quickly in your palms. Once you stop applying pressure to the mixture, it will flow out of your hands like a liquid.

Try filling a pie plate with a think layer of Oobleck and then slapping the surface with your open hand. Because of the dilatant properties, becoming more viscous when a force is applied, the liquid will all stay in the plate. Try the same experiment with water and compare the results!

If you have a lot of cornstarch and a small pool (or a large one like in the video), you can supersize this experiment. Since the liquid becomes more viscous when pressure is applied, you can actually walk or run on the surface without sinking. Of course, once you stop moving, you will begin slowly sinking into the liquid.